Tunable Fabry-Perot etalons are used as bandpass filters. The basic configuration includes at least two mirror structures defining a resonant cavity, in which the optical length of the cavity can be modulated.
In one application, the Fabry-Perot (FP) tunable filters are used to monitor wavelength division (WDM) systems. A mode of the tunable filter is scanned across the signal band of a WDM signal. A detector is used to detect the optical power during these scans. In other applications, the FP filters are used in channel add/drop devices. Commonly, two cavity Fabry-Perot filters are useful in these situations because of their top hat filtering profile.
Sometimes reference signal sources are used in conjunction with FP filters. The signal source generates a reference signal in a reference band, the reference signal having temporally stable and known spectral features. The tunable filter is scanned across these features. This provides information concerning the absolute passband of the tunable filter during a subsequent scan in the signal band.
In some applications, a relatively wide bandwidth FP filter is required. This can occur various situations. Sometimes the signal band is spectrally wide or scanning across two separate signal bands, such as the L and C bands in the ITU grid, is required. In other applications, the reference band is spectrally separated from the signal band. In still other implementations, the spectral distance between bands may even be greater than the free spectral range of the filter. In such cases, a mode in one order of operation of the tunable filter is used to scan one band, whereas a mode in another order of operation is used to scan the other band.
One problem that arises when fabricating FP filters that must accommodate these wide spectral bands concerns the construction of the mirror structures, and specifically the optical coatings. The mirror structures are typically fabricated from dielectric thin film coatings of alternating high and low index dielectric films. Thickness of these films is controlled to be approximately one quarter of a wavelength of the radiation to be reflected. This wavelength dependency conflicts with the need for a wide bandwidth. Generally, it is difficult to fabricate a thin film dielectric mirror that operates across a bandwidth of greater than about 300 nanometers (nm) in the communications wavelengths around 1550 nm.
To address these issues, the present invention is directed to an FP filter that comprises at least two mirror structures defining a resonant cavity. This filter is tunable by modulating an optical distance between the mirror structures. To accommodate a wide bandwidth of operation or accommodate two discrete spectral bands, the mirror structures are made from two stacked, single-band thin film mirrors.
In the current implementation, one of the mirror structures is located on a deflectable micro electromechanical system (MEMS) structure. Specifically, a MEMS membrane is deflected by electrostatic forces to thereby tune the optical distance between the mirror structures. Also, the mirror structures preferably define a curved/flat optical cavity.
In the preferred embodiment, each single band mirror is preferably constructed from multi-layer thin film dielectric mirror coating. One of the mirrors is designed to be reflective in a first spectral band whereas the second mirror is designed to be reflective in a second, different spectral band. For example, the first spectral band is a reference band and the second spectral band is a signal band, in one implementation.
In more detail, the mirror structures each comprise a substrate. The first mirror of each structure is deposited on the substrate, with an index matching coating between the substrate and the first mirror. The second mirror is stacked on the first mirror. The mirrors are symmetric relative to each other, such that the index of a first mirror in the second spectral band has an effective index of about one. In contrast, the second mirror has an effective index of about one in the first spectral band.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.